9. Time To Monkey Shine
The Killing Joke's failed-comedian-meets-vat-of-acid story is the closest thing that exists to an official Joker origin tale, but other explanations have been tabled by comic book writers over the years.
Among the most memorable is the Time to Monkey Shine arc from 2013, which introduced Mister J's abusive Aunt Eunice, a contender for one of the most evil characters ever to set foot in the DC universe.
There was no botched robbery is this origin story, but there was a chemical bath of sorts. Mister J's wicked aunt scrubs his skin with bleach until it takes on a white hue, along with starving him, beating him and taking away his only companion, a toy gorilla.
So damaged by these early-life experiences, the Joker kidnaps a baby gorilla from Gotham Zoo in later life so he can treat it to the kind of childhood he was denied and mould it into a sidekick of sorts. Which is all kinds of messed up.
Perhaps the ape-napping can be left out of the movie, but cherrypicking parts of Time to Monkey Shine and including Aunt Eunice flashbacks could add depth to the story, demonstrating how the protagonist's trauma stretches back to his childhood.